The present invention pertains to low-silica X zeolite industrial adsorbents which are particularly effective for the non-cryogenic separation of gases from air.
The separation of gases from air with faujasite has been envisaged practically since the synthesis of the zeolites was made possible (notably, Milton's studies) without the calcic faujasites having been shown to have a particular industrial value for this purpose.
The faujasites constitute a group of mineral species characterized by their crystallographic topographic structure. This topic received excellent coverages in the book by Donald W. Breck "Zeolite Molecular Sieves", John Wiley and Sons, 1974, p. 92 ff. Lowenstein's law requires an Si/Al ratio greater than or equal to 1. It is customary to distinguish:
(i) the X faujasites with Si/Al&lt;1.5, and PA0 (ii) the Y faujasites with Si/Al&gt;1.5. PA0 (i) a high nitrogen adsorption capacity, and PA0 (ii) a large difference between the nitrogen adsorption capacity and the oxygen adsorption capacity.
However, the conventional X faujasites have an Si/Al ratio .ltoreq.1.2. The physical characteristics of faujasites (exchange capacity, nitrogen adsorption, etc.) evolve in relation to the Si/Al ratio, and the traditional methods for producing X faujasites are not successful for the preparation of zeolites with an Si/Al ratio close to 1. Specifically, from a purely sodium medium, NaA zeolite is produced and it is necessary to introduce a certain amount of potassium ions in order to synthesize the faujasite structure. The synthesis of such zeolites with an Si/Al ratio=1 was described in British Patent No. 1,580,928.
The functional characteristics required for an adsorbent for the non-cryogenic separation of gases from air are the following:
Industrial adsorbents for the separation of gases from air are characterized by their adsorption of nitrogen at 25.degree. C. under 1 bar [C.sub.N2 ], expressed in liters (TNP) of nitrogen adsorbed per kilogram of adsorbent, and by the ratio of the nitrogen to the oxygen adsorption capacities [.alpha..sub.N2/O2 ] under 1 bar. These capacities are measured using techniques, which are well known by those skilled in this art, in an apparatus under vacuum either gravimetrically or volumetrically after degassing under vacuum.
The adsorbents which have been attainable industrially until now are characterized by a nitrogen adsorption capacity [C.sub.N2 ] of at most 13 liters/kg and an adsorption selectivity [.alpha..sub.N2/O2 ], ratio of the respective nitrogen and oxygen adsorption capacities, of circa 3. These values are for agglomerates with an X zeolite level of circa 80% or sieves without binder with an X zeolite level of circa 80%.